1. Field of the Invention
The present invention relates to a developer for forming an electrophotographic image, and to an image forming method using the developer.
2. Discussion of the Background
As a conventional electrophotographic full-color recording method, a method of developing each color image with plural photoreceptors is known.
This method separately forms each color toner image through each latent image forming process, developing process and transfer process, has a small difference between a single color image forming speed and a full color image forming speed, and has an advantage of being capable of meeting specifications for high speed printing. However, since this method forms each color toner image on a separate photoreceptor and layers each color toner layer (color lapping) to form a full color image, when each color toner has different properties such as chargeability, each color toner image has a different developed toner quantity, resulting in large variation of a secondary hue, i.e., deterioration of color reproducibility. In addition, this is same when each photoreceptor has a different chargeability and a different potential after irradiated.
Further, since this method transfers and fixes each toner image formed on the plural photoreceptors on an image forming substrate, when each color toner has a different adherence thereto, the color toner is not stably fixed, resulting in deterioration of color reproducibility. Since a conventional toner prepared by a pulverizing method nonuniformly includes materials dispersed therein in a fracture cross section thereof, surface properties thereof are difficult to fix, and each color toner is difficult to have a stable developed toner quantity and a uniform adherence to the image forming substrate. Accordingly, each color toner has a different developability and transferability, resulting in deterioration of color images. Particularly, since each color toner has a different transferability, color reproducibility thereof tends to deteriorate and an incomplete transfer thereof tends to occur.
Although an intermediate transferer prevents background fouling from directly transferring to a recording medium such as a paper when the photoreceptor has the background fouling, the toner transfers from the photoreceptor to the intermediate transferer and to the recording medium therefrom, and therefore the toner is difficult to have sufficient transferability.
On the other hand, although a conventional polymerized toner prepared by a suspension polymerization method has uniform surface properties, the toner has a spherical shape and has high adherence to the photoreceptor and image forming substrate. Particularly, cleanability of an elastic blade cleaning the photoreceptor and intermediate transferer tends to deteriorate.
As mentioned above, a tandem image forming method is difficult to stably produce high-quality color images for long periods.
Each color toner for use therein needs to have a stable developed toner quantity and a uniform adherence to the photoreceptor and image forming substrate.
Japanese Laid-Open Patent Publication No. 2001-318482 discloses a toner for use in a tandem image forming method, wherein one of the following conditions is satisfied:
(1) the toner has a shape factor having a variation coefficient not greater than 16% and a number particle diameter distribution having a number variation coefficient not greater than 27%; (2) the toner includes a polled toner in an amount not less than 50% by number and has a number particle diameter distribution having a number variation coefficient not greater than 27%; and (3) the toner includes a toner having a shape factor of from 1.2 to 1.6 in an amount not less than 65% by number and has a shape factor having a variation coefficient not greater than 16%.
Japanese Laid-Open Patent Publication No. 2002-244400 discloses a toner for use in a tandem image forming method, wherein the toner has a flat shape and toner images formed thereof are overlaid on an intermediate transferer having an adherence.
Japanese Laid-Open Patent Publication No. 2002-304025 discloses a toner, wherein a relationship between an average circle-equivalent diameter of the toner on a surface of a photoreceptor after an electrostatic latent image is developed and a value derived by dividing a standard deviation thereof with the average circle-equivalent diameter is specified; the toner has a volume-average particle diameter of from 2 to 7 μm; the toner has a volume variation coefficient not greater than 22; the toner includes a toner having a shape factor of from 1.2 to 1.6 in an amount not less than 60% by volume; and the shape factor has a variation coefficient not greater than 18%, and an image forming apparatus wherein a DC voltage overlapped with an AC voltage is applied to a photoreceptor.
Any of these methods specify a particle diameter distribution and a shape of a toner, but even a two-component developer including such a toner does not have stable developability and developing uniformity.
Because of these reasons, a need exists for a toner having good developability and transferability, which is capable of stably producing high-quality color images having good solid image uniformity and thin line reproducibility for long periods in a tandem color image forming method.